The present invention relates to improvements in interference suppression systems and more particularly to improved techniques for sampling the interference in a multiple signal environment.
Conventional interference suppression systems, for example, side-lobe canceller systems, act to isolate signals received by the main lobe of an antenna from interference that may be present in the side-lobes. Usually, such systems as exampled by U. S. Pat. 3,202,990, employing auxiliary antennas no sample the interference environment, have worked well to eliminate interference received by the main antenna when signals from only one interference source are present and supplied to a single canceller loop. However, when multiple interference sources are present, conventional systems have been less effective in reducing interference even where multiple canceller loops have been used.
In theory, if N independent interference sources are present forming a signal environment, the interference signals may be cancelled from the desired signal in a main antenna by multiple canceller loops fed by inputs from N separate auxiliary antennas. In practice, however, it has been found that conventional side-lobe canceller systems, using multiple auxiliary antennas whose positions are fixed relative to a main antenna, have limited effectiveness against multiple interference sources even when the number of auxiliary antenna-canceller loop combinations equals or slightly exceeds the number of interference sources present. Generally, the auxiliary antennas are situated symmetrically about the main antenna to provide coverage in all directions. However, the performance of such a multiple loop canceller system has been found to vary greatly with the direction of jammer attack and jammer spacing. Significantly degraded performance has been particularly noted in systems subject to interference from a plurality of closely spaced jammers (interference sources) where two in line auxiliary antennas receive nearly the same relative phases. Since canceller loops cannot change the relative phases of their received signals in such a situation, the action of the canceller loops are redundant and their combined performance is substantially equivalent to that of only one antenna-loop combination. As a result, present systems are incapable of adequately protecting receiving systems against sophisticated multiple interference countermeasure techniques.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a new technique for improving the performance and versatility of a canceller system in a multiple signal environment.